If you want to get into an already-sealed envelope without ripping it apart, all you need to do is pop it in the freezer for an hour or two.

Whether you forgot to include something with your letter or you have more nefarious schemes, this method should open up an envelope as if it were never sealed shut—with less mess than the more popular steaming method. Just throw it in the freezer for an hour or two (eHow recommends putting it in a plastic bag to keep it ice-free), then take it out and immediately open it with a knife or letter opener. Yes, you still have to open it, but if all went well, you should be able to re-set the adhesive with moisture afterwards, once it's reached room temperature.